Compact solid-state laser

ABSTRACT

A compact optically-pumped solid-state laser designed for efficient nonlinear intracavity frequency conversion into desired wavelengths using periodically poled nonlinear crystals. These crystals contain dopants such as MgO or ZnO and/or have a specified degree of stoichiometry that ensures high reliability. The laser includes a solid-state gain media chip, such as Nd:YVO 4 , which also provides polarization control of the laser; and a periodically poled nonlinear crystal chip such as PPMgOLN or PPZnOLT for efficient frequency doubling of the fundamental infrared laser beam into the visible wavelength range. The described designs are especially advantageous for obtaining low-cost green and blue laser sources.

PRIORITY INFORMATION

This application claims priority from co-pending, commonly assigned U.S.Provisional Application US60/795,212, filed Apr. 27, 2006.

BACKGROUND OF THE INVENTION

Compact, efficient, and low-cost laser sources in the visible andultraviolet spectral regions have long been desired for a variety ofapplications. These applications include laser-based projectiondisplays, optical storage, bio-analytical instrumentation, semiconductorinspection and spectroscopy. Semiconductor lasers, which provide alow-cost, compact, and efficient platform, rely on material systems suchas InGa(AI)P that lase most efficiently in the near-infrared spectralregion. Efficient operation down to ˜650 nm (red color) can be achievedwithout serious technological challenges and some semiconductor laserdesigns can be extended down to ˜635 nm with however decreasingefficiency and reliability. On the shorter wavelength side of thevisible region, GaN systems have been developed in recent years andlasers in the violet (˜400 nm to ˜445 nm) spectral range have beencommercialized. However, achieving wavelengths >470 nm in an efficientand reliable way represents a nearly insurmountable challenge. Thus, themajority of the visible spectrum (i.e., from ˜470 nm blue to ˜635 nmred) does not currently have an efficient semiconductor laser solution.

Of these colors (wavelengths), the absence of green is perhaps the mostnotable since this color corresponds to the peak sensitivity of thehuman eye. Indeed, no direct solution for a green semiconductor laser iscurrently available. The indirect solution, commercialized since the1990s, has been based on nonlinear frequency doubling (also known assecond-harmonic generation, or SHG) of neodymium (Nd)-based solid-statelasers, such as Nd:Y₃Al₅O₁₂ (Nd:YAG) or Nd:YVO₄. These solid-state gainmaterials can be pumped by infrared semiconductor lasers (e.g., at ˜808nm) and produce laser radiation at ˜1064 nm wavelength. This 1064 nmradiation can then be frequency doubled into the green 532 nm wavelengthusing nonlinear crystals such as Potassium Titan Phosphate (KTP) orLithium Borate (LBO). A similar technique can be used to obtain the bluecolor, e.g. 473 nm by frequency-doubling a 946 nm solid-state laser. Areview of such approaches can be found in the book by W. P. Risk, T. R.Gosnell and A. V. Nurmikko, “Compact Blue-Green Lasers”, CambridgeUniversity Press (2003). Furthermore, the low-cost platform can beachieved by using so-called microchip technology, where the gain chipand non-linear crystal are bonded to form a monolithic laser cavity. Themicrochip concept was apparently first proposed by Mooradian (U.S. Pat.No. 5,365,539).

However, the currently available microchip lasers lack the efficiencyand flexibility required in many applications, especially at higherpower levels, e.g. from several hundred milliwatts up to several Watts.This is mainly due to the frequency conversion inefficiency ofconventional nonlinear materials such as KTP. In order to obtainhundreds of milliwatts of green color from a KTP-based microchip laser,one has to provide a significant power margin for the fundamentalinfrared laser, which imposes thermal, size, and cost limitations on theoverall laser system design. Furthermore, traditional bulk nonlinearmaterials such as KTP are restricted as to their scope of frequencyconversion. For example, KTP is used for frequency doubling into thegreen color but cannot be practically used for frequency doubling intothe blue color, so one has to search for different nonlinear materialswith their own limitations in efficiency, reliability, and cost.

Laurel (U.S. Pat. No. 6,259,711), proposed that many of such limitationscan be overcome by the use of periodically poled nonlinear crystals.These crystals can be engineered to provide high nonlinearity for thedesired conversion wavelength. Therefore, such a laser designimplemented in a microchip architecture, could address many of therestrictions associated with conventional bulk nonlinear materials.

However, embodiments of that invention suffer from serious limitations,which, to our knowledge, have prevented commercialization of thisplatform, and, to this day, visible wavelength microchip lasers continueto rely on bulk nonlinear materials such as KTP and KNbO₃, the lattermaterial being used to produce the blue color (see, e.g., World PatentApplication WO2005/036,703). The origin of such limitations lies in thechoice of periodically poled nonlinear crystals proposed in Laurell'sinvention, i.e. KTiOPO₄ (KTP), LiNbO₃ (LN), and LiTaO₃ (LT). Thesematerials possess high nonlinearity and can be readily poled intoperiodic structures for frequency doubling. However, the practical useof these materials is very limited. Like bulk KTP, periodically poledKTP can only perform well at low power levels (a few milliwatts orpossibly even tens of milliwatts in the visible) but suffers frompassive and induced absorption (“gray tracking”) at higher power levels.In addition, KTP crystal production is not easily scalable to massproduction quantities at low cost as is required by some applicationssuch as consumer-electronics displays. LiNbO₃ and LiTaO₃ are scalable tohigh-volume production and can be readily periodically poled, but sufferfrom visible-light-induced degradation (“photo-refractive damage”) thatmakes it impossible to use these crystals to produce even milliwatts ofvisible light without severe degradation. The photo-refractive damagecan be reduced at elevated temperatures (>150° C.). However, thisrequires using ovens for maintaining the nonlinear crystals at ahigh-temperature. Such ovens are incompatible with a low-cost, efficientlaser fabrication, especially in a microchip geometry. Thus, the laserdesigns described by Laurell, cannot be implemented in a high-power,low-cost, compact, and efficient architecture. Similarly, Brown (USPublished Patent Application 2005/0,063,441), proposed designs forcompact laser packages, which would appear to be suitable for low-costapplications. However, the Brown teaching is still centered onconventional nonlinear materials such as KTP and LBO. The possible useof PPLN and PPKTP is mentioned but it is not taught how one can overcomethe limitations of these crystals, especially their afore mentionedreliability limitations.

It is known that congruent LiNbO₃ and LiTaO₃ suffer fromphoto-refractive damage due to visible light, and several ways toovercome this problem have been proposed. The high-temperatureoperation, mentioned above, partially solves the problem, but is notsuitable for most applications. Another proposed solution is doping thecongruent material during the crystal growth to suppressphoto-refractive damage mechanisms (T. Volk, N. Rubinina, M. Wöhlecke,“Optical-damage-resistant impurities in lithium niobate,” Journal of theOptical Society of America B, vol. 11, p. 1681 (1994)). Growing bulkcrystals with a high degree of stoichiometry has been proposed asanother method to suppress photo-refractive damage (Y. Furukawa, K.Kitamura, S. Takekawa, K. Niwa, H. Hatano, “Stoichiometric Mg:LiNbO₃ asan effective material for nonlinear optics,” Optics Letters, vol. 23, p.1892 (1998)).

However, none of the prior art workers have taught a means of achievinga high output power, stable ambient temperature operable frequencydoubled laser suitable for producing green and blue light in a low-cost,mass-manufacturable design. We have found that if periodically poledLiNbO₃ or LiTaO₃ crystals are within 0.05% of stoichiometric they do notrequire any dopant to be stable at high output powers of up to 500 mW.For crystals that are within 0.6% of stoichiometric, doping with fromabout 0.1 to about 0.6 mole % of ZnO or MgO achieves substantially thesame beneficial results as are obtained with stoichiometric,periodically poled LiNbO₃ or LiTaO₃ crystals. The present inventionteaches a compact, efficient, and low-cost frequency-converted laserbased on periodically poled materials that contain as dopants MgO or ZnOand/or have a specified degree of stoichiometry that ensures highreliability for these materials. ZnO or MgO-doped stoichiometric LiNbO₃and LiTaO₃ are very different materials from their congruentcounterparts and their altered ferroelectric properties make thesematerials exceedingly difficult to pole into the short-periods,several-micron-length domains required for frequency conversion into thevisible spectral range. The technological challenges in producingperiodically poled ZnO or MgO-doped and stoichiometric LiNbO₃ and LiTaO₃have recently been overcome and these new materials shown to bemanufacturable. Crystals with poling periods suitable for laserconversion into blue, green, and longer wavelength ranges have beenproduced and the technology for such production process is described incopending, commonly assigned Published US Patent Application2005/0,133,477 the disclosure of which is hereby incorporated herein bythis reference.

In short, known technical approaches cannot provide a reliable,cost-effective, and compact frequency converted laser. The presentinvention solves this problem and discloses a low-cost, efficient, andreliable solid-state laser architecture that is based on periodicallypoled LiNbO₃ or LiTaO₃ that contain dopants such as MgO or ZnO and/orhave a specified degree of stoichiometry that ensures high reliabilityfor these materials. The present invention also describes a compact,efficient, reliable, and low-cost solid-state laser, frequency convertedinto wavelength ranges, not available through direct semiconductorlasers, i.e. into the blue, green, yellow, orange, and near-ultravioletwavelength regions, i.e., into wavelengths of about 275 nm to 635 nm.The present invention teaches a method of manufacturing compact andefficient visible or near-UV laser sources having output power levels ofat least several hundreds of milliwatts and even higher, which levelsare not achievable with existing technologies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a micro-chip embodiment of the present invention.

FIG. 2 shows an embodiment of the present invention with a gain mediumthat has no preferred polarization and a crystal with birefringentwalk-off.

FIG. 3 shows an embodiment of the present invention with a gain mediumthat has no preferred polarization and an intracavity Brewster surface.

FIG. 4 shows an embodiment of the present invention with a gain mediumthat has a preferred polarization and a waveplate for rotatingpolarization of a backward-propagating second-harmonic beam

FIG. 5 shows an embodiment of the present invention with recovery of abackward-propagating second-harmonic beam via a turning mirror

FIG. 6 shows an embodiment of the present invention with a folded cavity

FIG. 7 shows an embodiment of the present invention with a curved mirroron either side of the cavity

FIG. 8 shows an embodiment of the present invention with a saturableabsorber.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a preferred embodiment of the present invention. Thepump diode laser 1, emits a beam 2, for example, at a wavelength between800 and 900 nm, such as ˜808 nm or 885 nm for efficient absorption bythe gain material (element) 8. The beam 2 is frequently astigmatic andbeam-shaping optics 3 are advantageously used to convert the pump beam 2into the beam 4 so that the beam 4 forms a circular cross-section of thedesired diameter on the surface 7 of gain medium 8. This type of pumpingarrangement is known in the art and can efficiently overlap the pumparea in the gain element with the intracavity circulating beam, whichmust be a single-spatial mode (or TEM₀₀) for efficient nonlinearfrequency doubling. A suitable diameter for the pump spot on the gainelement 8 is in the range of 50 to 300 microns. The beam-shaping opticscan be a micro-lens, a gradient-index lens, or a combination of suchoptical elements. When efficiency can be sacrificed in favor ofsimplicity and compactness, the beam-shaping optics 3 can be eliminated.Another part of assembly 3 may be a volume Bragg grating used to narrowdown the spectral emission of diode laser 1. Narrowing down the spectraloutput of the pump laser may be beneficial for the efficiency of thelaser system. Methods to achieve such spectral narrowing have beendescribed, e.g., in the paper by L. Glebov. “Optimizing and StabilizingDiode Laser Spectral Parameters.” Photonics Spectra, January 2005.

However, producing high laser source efficiency is a key benefit of thepresent invention. To maximize efficiency, we use a transparent opticalmaterial 6, which has a high thermal conductivity such as sapphire,undoped YVO₄, or undoped Y₃Al₅O₁₂(YAG). Thus, element 6 is bonded to thegain element 8 and acts as a heat sink. The surfaces 5 and 7 are coatedfor high transmission at the pump laser wavelength, e.g., 808 nm. Thecoating of surface 7 also provides high reflectivity at the fundamentallaser wavelength, such as 1064 nm, and serves as the first mirror of thesolid-state laser cavity. The coating may be selected for lasing in thedesired wavelength supported by the solid-state material 8, e.g. 1342nm. In this instance, care must be taken to reduce the reflectivity ofthis mirror 7 or the second cavity mirror 12 at the dominant lasertransition wavelength (1064 nm in the case of a Nd:YVO₄ pump laser ).Some examples of optically transparent heat sink material suitable foruse as the element 6 include sapphire, undoped YVO₄, and undoped YAG. Ofthese elements, sapphire is the most efficient for heat sinking due toits high thermal conductivity and good thermal expansion match toNd:YVO₄. In lower-power versions of this laser design (<1 W of absorbedpump power), traditional heat sinking methods such as mounting the gainelement on a copper or another high-thermal-conductivity metallic mountare acceptable and are also within the scope of this invention.

Gain medium 8 is preferably a Nd-doped element with a higher gain in oneaxis, such as Nd:YVO₄ or Nd:GdVO₄ so that the element 8 provides bothgain and polarization control for the laser cavity. The level of Nddoping for maximizing laser efficiency in this invention will typicallybe in the range of 0.5% to 3% atm (atomic percent). The element 8 alsoprovides the transverse mode control in the otherwise flat-flat lasercavity through gain-guiding and thermal lensing effects.

The nonlinear crystal 10 is a periodically poled nonlinear crystal thatbelongs to the family of doped or stoichiometric nonlinear materialsthat ensure reliable crystal operation at both fundamental wavelength(namely, near-infrared) and at the second-harmonic wavelength(typically, visible). Specifically, these materials comprise PPMgOLN(periodically poled MgO-doped congruent LiNbO₃), PPMgOLT (periodicallypoled MgO-doped congruent LiTaO₃), PPZnOLN (periodically poled ZnO-dopedcongruent LiNbO₃), PPZnOLT (periodically poled ZnO-doped congruentLiTaO₃), PPSLN (periodically poled stoichiometric lithium niobate) orPPSLT (periodically poled stoichiometric lithium tantalate) The levelsof doping and stoichiometry are selected to suppress optical degradationeffects such as photo-refractive damage and visible-light-inducedinfrared absorption (also knows as GRIIRA and BLIIRA for green and bluelight, respectively). A recent discussion on this subject can be foundin the paper by Y. Furukawa, K. Kitamura, A. Alexandrovski, R. K. Route,M. M. Fejer, G. Foulon, “Green-induced infrared absorption in MgO dopedLiNbO₃,” Applied Physics Letters, vol. 78, p. 1970 (2001). Methods formass manufacturing such periodically poled crystals are described by S.Essaian, one of the co-inventors of the present invention, in PublishedUS Patent Application 2005/0,133,477 assigned to the same assignee asthe present application.

The poling period of the nonlinear crystal 10 is chosen to maximize theefficiency of the second-harmonic generation of the fundamental beam.For example, the poling period of PPMgOLN for frequency doubling of 1064nm into 532 nm is approximately 7 micron. The effective nonlinearcoefficient for such a material is about 16 pm/V and can be as high as20 pm/V when perfect grating structure and material stoichiometricuniformity are achieved. The high nonlinearity and high reliability ofthe nonlinear crystals are key advantages of the laser system of thepresent invention. Since the efficiency of nonlinear conversion scaleswith the square of the nonlinear coefficient, the use of such materialsas PPMgOLN instead of traditional materials such as KTP (˜3.5 pm/V forconversion into the green wavelength) or LBO (˜1 pm/V) allowsconstructing more compact, less power consuming, and higher power outputsystems than traditional bulk materials allow. As a result of the highefficiency of nonlinear crystals used in this invention (such materialsas PPMgOLN), short length of the green/blue laser microchip (and thecorresponding short laser cavity) can be designed as well. Thismicrochip laser design allows large longitudinal mode spacing and hastendency to generate single frequency radiation, which is requested bymany instrumentation applications. For example, if it is necessary thatlength of said green laser microchip has been restricted less than 1.3mm.

An additional advantage of using periodically poled materials comparedto, e.g. KTP, is that only a single polarization of the fundamental beamis necessary for the second harmonic generation process. In KTP (mostwidely used crystal for SHG into the green wavelength range), twoorthogonal polarizations at the fundamental wavelength have to beexcited in the crystal (this constitutes the so-called type-Ilphase-matched SHG) and this creates possibilities for the depolarizationof the intracavity laser beam, and, therefore, for the loss of bothpower and efficiency.

The use of optimal doping and stoichiometry for high reliability allowsmaking reliable laser products without the need of expensive andspace-consuming ovens to heat the nonlinear crystal to suppress itsdegradation. Finally, mass manufacturability of PPMgOLN and the othercrystals useful in the practice of the present invention allowsachieving mass production of compact visible lasers for high-volumeconsumer-electronics markets. It is important to point out that colorsnot available from direct semiconductor diode lasers can thus beachieved.

Using nonlinear crystals with non-periodic (chirped) or non-parallel(fan-out) poling patterns is also within the scope of the presentinvention. Another advantage provided by the high efficiency of thematerials of the present invention such as PPMgOLN is that they providedesign headroom. This means that the effective nonlinearity can betraded off for other parameters such a temperature or angular acceptancebandwidth for second-harmonic generation without significant penalty ingenerated second harmonic power. The reason is that the intracavitysecond harmonic generation is limited by the maximum amount of power thelaser can emit at the fundamental wavelength. This was described bySmith (R. Smith, “Theory of intracavity optical second-harmonicgeneration,” IEEE Journal of Quantum Electronics, vol. 6, p. 215,(1970)). After the laser limitation is reached, increasing crystalnonlinearity, length, or beam focusing can achieve no further increasein second harmonic power. While conventional bulk nonlinear crystalstypically never reach this regime in continuous wave laser operation,the high-nonlinearity periodically poled crystals of the presentinvention do reach it. As a result, this allows one to improve lasercost and performance by decreasing nonlinear crystal length, modifyingthe poling pattern, and, especially, by using a low-cost, monolithicmicrochip laser cavity assembly which provides some efficiencylimitations due to its inherent thermal gradients, even when the entireassembly is controlled as a whole. Thus, in a preferred embodiment,nonlinear crystal 10 is bonded to laser gain element 8, e.g. by mean ofchemically activated direct bonding (glue-free optical contact). Theinput surface 9 of the nonlinear crystal has a coating and opticalyfine-polished to ensure high transmission at the fundamental wavelengthand high reflection at the second harmonic wavelength. This arrangementalso prevents the generated visible light from entering the gainelement, which can be detrimental to the laser operation. It should benoted that glue(epoxy)-free bonding, which is preferred in the presentinvention, has seen significant progress recently, and, therefore, themonolithic assemblies disclosed here are readily manufacturable. Areview of direct bonding techniques can be found in the paper by C.Myaft, N. Traggis, and K. Dessau, “Optical contacting grows morerobust,” Laser Focus World, January 2005, p. 95 the disclosure of whichis incorporated herein.

The output surface 12 of the nonlinear crystal serves as the secondmirror of the cavity. Therefore, it is preferably coated for highreflection at the fundamental laser wavelength and for high transmissionat the second-harmonic wavelength. The longitudinal and lateraldimensions of the described arrangement are optimized for highefficiency as is known in the art of laser design. We have found thatthe nonlinear crystal length need not exceed 2-3 mm to obtain hundredsof milliwatts of power at the 532 nm (green color) wavelength. Theoptical beam 11 indicates the intracavity laser beam at the fundamentalwavelength. The beam illustrates the cavity mode propagating in thedirection away from gain element 8. The backward-propagating cavity modeoverlaps this forward-propagating beam and, therefore, is not shown.Similarly, second-harmonic beams are generated in both the forward andbackward directions. The backward-generated second-harmonic beam isreflected by the optical surface 9 and is recombined with theforward-generated second-harmonic beam so that a single beam 13 exitsthe laser cavity.

It should be noted that because both forward- and backward-generatedsecond harmonic beams are coherent (i.e., have a definite phaserelationship) with each other, they could optically interfere with eachother, somewhat reducing the efficiency of nonlinear conversion. Severalmethods to overcome this problem can be utilized in the practice of thepresent invention. One method is to control the crystal temperature (theoptimum point between maximizing interference to make it as close toconstructive interference as possible and maximizing nonlinearconversion efficiency. Suitable temperatures range from about 20° C. toabout 80° C. and can be easily achieved with the aid of a low-costresistive heater element positioned under the nonlinear crystal. Whenthe laser cavity is long enough to operate in multiple longitudinalmodes, another method is to rely on some longitudinal modesextinguishing themselves in the (partially) destructive interference,while other modes enhance the total second-harmonic output through aconstructive interference. In a multiple-longitudinal mode laser, thisis achieved automatically as the modes favored in constructiveinterference will be outcoupled most efficiently.

Yet another advantage for the microchip assembly of the presentinvention is being able to use periodically poled crystals that arethick enough to be handled easily and be bonded to other crystals. Untilrecently, the commonly accepted opinion was that such materials asPPMgOLN can at best only be poled in thin wafers (0.5 mm thick or less)for conversion into blue-green colors and not really be poled at all ina production, non-research environment. Now, by using the methoddescribed by S. Essaian in Published US Patent Application2005/0,133,477), it is possible to manufacture crystals as thick as 1 mmin high yield. This is a significant advantage for building a microchiplaser. Thus, by using this recent achievement in crystal technology, onecan obtain a new laser platform that surpasses existing platforms in itscapabilities: i.e., power, efficiency, reliability, and cost.

Regarding the invention embodiments illustrated in FIG. 2 and subsequentfigures, many elements and their functions are essentially the same asin the embodiment illustrated in FIG. 1. Therefore, the differences willbe highlighted in the subsequent description of these embodiments, whilesimilarities can be understood from the description of FIG. 1.

The embodiment of FIG. 2 is especially useful when the gain medium(element 15 in FIG. 2) does not have a preferred direction forpolarization to afford higher gain. A well-known example of such a gainmedia is Nd:YAG. One advantage of using Nd:YAG is that it can providelaser wavelengths, such as 946 nm, not available with Nd:YVO₄ orNd:GdVO₄. This is desirable for obtaining other colors by nonlinearfrequency conversion, e.g. blue color at a 473 nm wavelength. Gainmaterials may also be glass-based materials such as Yb:glass orNd:glass, Yb:YAG glass and also, other like based crystals and glasses,ceramics.

While many elements and technical methods described in the embodiment ofFIG. 1 apply to FIG. 2, the design of FIG. 2 provides polarizationcontrol via different means than the gain medium. Polarization controlis a necessary part of the laser design since the second-harmonicgeneration process is polarization-sensitive. In order to preserve thelow-cost, compact design concept of the present invention, a preferredembodiment of this invention utilizes an additional birefringent element16. Element 16 is a birefringent crystal, suitable for intra-cavitylaser design, cut at an angle to provide large walk-off between the twopolarizations supported by this crystal. An example of material suitablefor the use in element 16 is undoped yttrium vanadate (YVO₄). Thewalk-off in crystal 16 can be used to discriminate between the twopolarizations by, for example, using an aperture 18, which providehigher loss to the unwanted polarization. While the illustration in FIG.2 shows separate elements 15 (gain crystal), 16 (polarization controlcrystal for creating walk-off), 18 (aperture), and 19 (nonlinearcrystal), they can also be joined in a monolithic assembly. In thiscase, a significant walk-off can be designed in so that the aperture canbe aligned passively, i.e. before the laser is turned on.

Another way to discriminate between the two polarizations is to use acurved mirror or a lens on the right of the nonlinear crystal (not shownin the figure) so that one of the polarizations is walked out ofalignment with respect to the optical axis defined by the lens or themirror on one side and by the gain aperture on the other side. Theconcept is essentially similar to the embodiment with an aperture inthat it provides higher loss to the unwanted polarization. Otherelements and coatings in the embodiment shown in FIG. 2 are similar tothose in FIG. 1.

The design of FIG. 3 is similar to the design of FIG. 2 in that it isparticularly useful when the gain medium (element 15 in FIG. 2) does nothave a preferred direction for polarization with higher gain. To controlthe laser polarization for efficient nonlinear frequency doubling, thisdesign relies on the intracavity Brewster surface 52, which can be leftuncoated. One way to obtain a Brewster surface in the cavity withoutadding extra elements is to cut the gain crystal 51 at the Brewsterangle. Brewster surfaces have high transmission for p-polarized lightand lower transmission for s-polarized light. This fact can be used totilt the gain crystal at the appropriate angle to form the laser cavity.The crystal shown in FIG. 3 appears thinner than in other figures. Thisis to illustrate the fact that the thinner (wafer) cross-section of theperiodically poled crystal will typically be in the plane of thedrawing, when a Brewster surface is present. Designs similar to the oneillustrated in FIG. 3 have been used in the past (see, e.g., WorldPatent Application WO2005/036,703), but did not take advantage of thehigh-reliability, periodically poled crystals taught in the presentinvention.

It must be understood that FIG. 3 illustrates only one possible scenarioof component arrangement with an intracavity Brewster surface. As inFIG. 1, this design can be monolithically built, e.g., by cutting thesurface 54 of the nonlinear crystal 10 at an angle and joining the gainelement and the nonlinear crystal. In this case, the Brewster angle cutis designed for the interface to be between optical materials 51 and 10and not between either of these materials and air.

The embodiment shown in FIG. 4 illustrates and addresses theoptimization of second-harmonic power extraction. As was discussed inthe description for FIG. 1, the second harmonic light is generated intwo opposite directions of propagation. In many cases thebackward-generated beam can be recombined with the forward-generatedbeam via a high-reflectivity mirror coating for the backward-generatedbeam and possible destructive interference between the two beams can beavoided by thermal adjustments by using multi-longitudinal modeoperation. However, in some cases it is more efficient to use the designshown in FIG. 4.

Element 23 is a waveplate (made, e.g., from quartz) that rotatespolarization of both fundamental and second-harmonic beams. In thisdesign, the waveplate is selected so that the polarization of thefundamental beam is rotated by 90 degrees after a single pass, and thepolarization of the second harmonic beam is rotated by 45 degrees aftera single pass. Waveplates of this type are called dual waveplates andare commercially available. Surface 23 is anti-reflection coated forboth the fundamental and second harmonic beams. Surface 22 isanti-reflection coated for the fundamental beam, and coated for highreflection for the second harmonic beam. Since the fundamental lighttraverses the waveplate twice in one cavity round trip, it does notchange its polarization and thus the waveplate does not disturb theoperation of the fundamental laser. However, the second harmonic light,which also traverses the waveplate twice, changes its polarization tothe orthogonal one and returns back through the nonlinear crystal 10(the surface 24 is now anti-reflection coated for both fundamental andsecond harmonic beams) without interference with the forward-generatedsecond-harmonic beam. This design is especially useful in applicationsfor which the polarization of the output second harmonic beam is notcritical. One such application is using the laser of the presentinvention for projection displays, which are based on digital lightprocessing technology.

The embodiment of FIG. 5 illustrates another method to extract thebackward-generated second-harmonic beam when the use of waveplates inundesirable. The extraction is now done via a coated tuning mirror 28,which has a high reflection for the second harmonic light and hightransmission for the fundamental light. One instance when the turningmirror design of FIG. 5 may be preferred over the waveplate design ofFIG. 4 is when the laser polarization is not locked by the gain element6, e.g. when Nd:YAG is used. In this case, it is possible to also designpolarization discrimination in the coatings of element 28 so that thelaser is operating only in the desired polarization, providing efficientsecond-harmonic conversion. The re-directed, backward-generatedsecond-harmonic beam 30 may be re-directed again by another mirror sothat it propagates in the same direction as the forward-generatedsecond-harmonic beam. Unlike the design of FIG. 4, this design willproduce a linearly-polarized second-harmonic beam. This is desirable forapplications such as using the laser for projection displays, based onliquid-crystal spatial light modulators, such as LCD or LCOS.

The embodiment of FIG. 6 combines the forward- and backward-generatedsecond harmonic beams by reflecting the forward-generated beam from thesurface 37 of the nonlinear crystal 36. The coated glass plate 35 ispreferably oriented at an angle of 45 degrees with respect to the gaincrystal surface and is coated for high reflection at the fundamentallaser wavelength and high transmission at the second-harmonicwavelength. A single, linearly polarized second-harmonic beam 39 isoutcoupled from surface 35. As mentioned previously, having a tiltedsurface in the cavity makes it easier to discriminate polarizations bydesigning polarization-selective coatings. This is advantageous for gaincrystals that do not define laser polarization direction, such as aNd:YAG gain crystal. Like other embodiments of this invention, thisdesign is modular and can be combined with the concepts illustrated inother embodiments, e.g. with the “waveplate design” of FIG. 4 configuredto rotate polarization of the second-harmonic beam. One embodiment is touse the waveplate to complete the cavity below the surface 36, whichwill be suitably dual-band anti-reflection coated in this case.

The configuration shown in FIG. 7 is similar to the design of FIG. 1 andcan be combined with the designs of FIGS. 2-6. The different element inthis embodiment is a curved mirror 13, which has high reflectivity atthe fundamental laser wavelength and high transmission at the secondharmonic wavelength. This design may be somewhat higher-cost than theother designs illustrated but can be used for higher-power applicationswhen thermal lens stabilization of the cavity transverse mode becomesless efficient than it is at lower power levels. Note that the curvedmirror can be used on another side of the cavity as well in anon-monolithic arrangement.

The invention embodiments in FIGS. 1-7 illustrate low-cost and compactlaser designs for continuous-wave (cw) operation. As will be apparentfrom FIG. 8, a compact and low-cost design for a pulsed (passivelyQ-switched or passively mode-locked) operation can also be obtainedwhile enjoying all the advantages of the nonlinear crystals described inthe present invention. FIG. 8 illustrates the design of FIG. 5, modifiedfor operation with a saturable absorber 71. Element 71 is suitably asolid-state or semiconductor saturable absorber. An example of asolid-state saturable absorber is Cr⁴⁺:YAG (chromium doped yttriumaluminum garnet) and this and other saturable absorber crystals(V³⁺:YAG, Co²⁺:MgAl₂O₄). An example of a semiconductor based saturableabsorber is an epitaxially grown single quantum well or plural quantumwells (e.g., based on InGaAs material structure). The quantum wellabsorber may be grown together with an epitaxial mirror stack, alsoknown as a distributed Bragg reflector, or DBR. Similarly, thesolid-state saturable absorber crystal can be coated with a mirrorcoating to define the second cavity mirror. The methods of passiveQ-switching and mode locking are known in the art of laser design andare described e.g., in the following references: R. Paschotta and U.Keller, “Ever higher power from mode-locked lasers,” Optics andPhotonics News, p. 50, May 2003; D-H Lee et al., “Intracavity-doubledself-Q-switched Nd,Cr:YAG 946/473 nm microchip laser,” Chinese PhysicsLetters, vol. 19, p. 504 (2002); J. J. Zayhowski, “Passively Q-switchedmicrochip lasers and applications,” Rev. Laser Eng., vol. 26, p. 841(1998). Furthermore, the saturable absorber and the gain element can becombined in a single element 26, e.g., by co-doping YAG crystal with Ndand Cr. The pulsed embodiment may be advantageous in applications wherecw operation is not required. An additional advantage of the pulsedlaser configuration is a much higher peak power in a pulse compared tothe average power at the fundamental wavelength. This allows to furtherincrease the efficiency of second harmonic generation and relaxes thetemperature tolerance on the periodically poled nonlinear crystal.

Furthermore, a design with active Q-switching can also be realizedwithout incurring a significant increase in the cavity cost andcomplexity. It is known that congruent periodically poled lithiumniobate (PPLN) and lithium tantalate (PPLT) can be used aselectro-optical Q-switch elements. A recent discussion on the subjectcan be found in the paper by Y. H. Chen, Y. C. Huang, Y. Y. Ling, and Y.F. Chen, “Intracavity PPLN crystals for ultra-low-voltage laserQ-switching and high-efficiency wavelength conversion,” Applied PhysicsB: Lasers and Optics, vol. 80, p. 889 (2005). Again, it is a preferredand advantageous embodiment of the present invention to use periodicallypoled nonlinear materials with optimized doping or stoichiometry onwhich a reliable and efficient commercial laser product can rely. Theillustration provided in FIG. 8 provides a compact, efficient, andreliable actively Q-switched laser, where the element 71 is now anotherperiodically poled nonlinear crystal used as an electro-optic Pockelscell element i.e., an electro-optic Q-switch. The teaching of thefollowing references is incorporated herein by this reference

-   1. W. P. Risk, T. R. Gosnell and A. V. Nurmikko, “Compact Blue-Green    Lasers”, Cambridge University Press (2003).-   2. A. Mooradian, “Microchip laser,” U.S. Pat. No. 5,365,539-   3. F. Laurell, “Laser,” U.S. Pat. No. 6,259,711-   4. T. Georges, “Laser diode-pumped monolithic solid-state laser    device and method of application of said device,” World Patent    Application WO2005/036,703-   5. T. Volk, N. Rubinina, M. Wöhlecke, “Optical-damage-resistant    impurities in lithium niobate,” Journal of the Optical Society of    America B, vol. 11, p. 1681 (1994).-   6. Y. Furukawa, K. Kitamura, S. Takekawa, K. Niwa, H. Hatano,    “Stoichiometric Mg:LiNbO₃ as an effective material for nonlinear    optics,” Optics Letters, vol. 23, p. 1892 (1998).-   7. D. C. Brown, “High-density methods for producing diode-pumped    microlasers,” US patent application 2005/0,063,441-   8. Spectralus Corporation Web Site: http://www.spectralus.com-   9. S. Essaian, “Method for the fabrication of periodically poled    lithium niobate and lithium tantalate nonlinear optical components,”    US patent application 2005/0,133,477.-   10. Y. Furukawa, K. Kitamura, A. Alexandrovski, R. K. Route, M. M.    Fejer, G. Foulon, “Green-induced infrared absorption in MgO doped    LiNbO₃,” Applied Physics Letters, vol. 78, p. 1970 (2001).-   11. R. Smith, “Theory of intracavity optical second-harmonic    generation,” IEEE Journal of Quantum Electronics, vol. 6, p. 215,    (1970).-   12. C. Myatt, N. Traggis, and K. Dessau, “Optical contacting grows    more robust,” Laser Focus World, January 2005, p. 95.-   13. L. Glebov. “Optimizing and Stabilizing Diode Laser Spectral    Parameters.” Photonics Spectra, January 2005.-   14. R. Paschotta and U. Keller, “Ever higher power from mode-locked    lasers,” Optics and Photonics News, p. 50, May 2003;-   15. D-H Lee et al., “Intracavity-doubled self-Q-switched Nd,Cr:YAG    946/473 nm microchip laser,” Chinese Physics Letters, vol. 19, p.    504 (2002);-   16. J. J. Zayhowski, “Passively Q-switched microchip lasers and    applications,” Rev. Laser Eng., vol. 26, p. 841 (1998).-   17. Y. H. Chen, Y. C. Huang, Y. Y. Ling, and Y. F. Chen,    “Intracavity PPLN crystals for ultra-low-voltage laser Q-switching    and high-efficiency wavelength conversion,” Applied Physics B:    Lasers and Optics, vol. 80, p. 889 (2005).

1. A compact and efficient microchip laser providing a frequency doubledvisible or near-ultraviolet output, comprising a pump laser whichprovides a pump beam at a selected wavelength and a laser microchip informing the laser cavity, further comprising: a) two mirrors, defined bycoated surfaces b) a solid-state gain element pumped by a semiconductordiode laser and disposed between said two mirrors, and c) a bulk,periodically poled nonlinear frequency doubling crystal also disposedbetween the two mirrors, said crystal comprising PPMgOLN, PPMgOLT,PPZnOLN, PPZnOLT, stoichiometric PPSLN, or stoichiometric PPSLT
 2. Thelaser of claim 1 wherein the gain element is a crystal that has a largergain along one of its crystalline axes
 3. The laser of claim 1 whereinthe gain element is Nd:YVO₄, Nd:GdVO₄, or Nd:YGdVO₄, Nd:YAG, Nd:YLF,Yb:glass, Yb:YAG, or Nd:glass
 4. The laser of claim 1 wherein the pumpbeam is delivered to the gain element via a microlens or agradient-index lens
 5. The laser of claim 1 wherein the pump beam isdelivered the gain element directly, without the utilization of beamshaping optics.
 6. The laser of claim 1 wherein the pump laser isspectrally narrowed with the aid of a volume Bragg grating
 7. The laserof claim 1 wherein the non-linear crystal is PPMgOLN, PPMgOLT, PPMgOSLN,PPMgOSLT, PPZnOLN, PPZnOSLN or PPZnOLT, PPZnOSLT and wherein the MgO orZnO dopant is present in an amount of 0.1 to 7 mol % and the LN and LTare congruent or are within 0.6% of stoichiometric.
 8. The laser ofclaim 1 wherein the periodically poled nonlinear crystal isstoichiometric LT (PPSLT) or LN (PPSLN)
 9. The laser of claim 1 wherethe laser cavity components are in the form of a monolithic assemblyachieved by glue-free, chemically activated direct bonding together theseparate elements
 10. The laser of claim 9 where the periodically polednonlinear crystal has length of ≦1 mm that said laser microchip has alength in the less than 1.3 mm, in order to create a short cavity forgeneration of single-frequency 532 nm or 473 nm radiation
 11. The laserof claim 1 where the laser cavity components are spatially separated andare mounted on a common platform.
 12. The laser of claim 1 where theoutput face of the nonlinear crystal forms the end mirror of the lasercavity and is coated for high reflection at the fundamental wavelengthand high transmission at the second harmonic wavelength thereof.
 13. Thelaser of claim 1 where the input face of the nonlinear crystal and/orthe input face of the gain element is coated for high reflection at thesecond harmonic wavelength to thereby collect the backward-generatedsecond-harmonic beam
 14. The laser of claim 1 where the gain element ismounted on an optically transparent material having a high thermalconductivity
 15. The laser cavity of claim 14 wherein said opticallytransparent material is sapphire, undoped YVO₄, or undoped YAG
 16. Thelaser of claim 1 wherein polarization control in a desired polarizationaxis is achieved by using a gain medium comprising Nd:YVO₄, Nd:GdVO₄, orNd:YGdVO₄.
 17. The laser of claim 1 further comprising: i) abirefringent element having a larger spatial walkoff between one of twosupported polarizations, and ii) an aperture, curved mirror or lenswhich imposes higher loss to the undesired polarization.
 18. The laserof claim 1 wherein polarization control is achieved by using anintracavity Brewster surface as part of either or both the gain elementor the periodically poled nonlinear crystal.
 19. The laser of claim 1wherein a dual waveplate is provided which leaves the fundamental beampolarization unchanged in a cavity round trip and rotates thepolarization of the backward-generated second-harmonic beam by 90degrees and reflects this second-harmonic beam to combine it with theforward-generated second-harmonic beam.
 20. The laser of claim 1 whereinan intracavity tilted and coated plate is provided to extract abackward-generated second-harmonic beam
 21. The laser of claim 20wherein an intracavity tilted and coated plate is provided to lock thepolarization of the pump laser at the fundamental wavelength
 22. Thelaser of claim 1 wherein the cavity arrangement is folded forpolarization control and for extraction of a second-harmonic beamthrough a tuning mirror
 23. The laser of claim 22 wherein thesecond-harmonic light is extracted from the tilted surface of theperiodically poled nonlinear crystal
 24. The laser of claim 1 wherein atleast one of the mirrors of the cavity is curved.
 25. The laser of claim1 wherein the pump laser is a continuous wave or pulsed laser
 26. Thelaser of claim 1 wherein the laser cavity is operated in a pulsedregime, obtained via passive Q-switching or passive mode-locking andfurther comprises a saturable absorber element in the cavity
 27. Thelaser of claim 26 wherein the saturable absorber element is Cr⁴⁺:YAG,V³⁺:YAG or Co²⁺:MgAl₂O₄.
 28. The laser of claim 26 wherein the saturableabsorber element is an epitaxially grown semiconductor structure
 29. Thelaser of claim 1 wherein the laser cavity is operated in an activelyQ-switched regime via electro-optic Pockels cell .